Magnetic core structure



Nov. 3, 1942. H v PUTMAN 2,300,964

MAGNETIC CORE STRUCTURE ATTO Nov. 3, 1942. H v, PUTMAN 2,300,964

MAGNETIC CORE STRUCTURE Filed Jan. 29, 1941 2 Sheets-Sheet 2 F1919.ffy/0. l 2, u n A "la a3-.94 32 ...r/33 N34 (-/77 mx ik, .1371* 4/38/.9

f l D l' 23 4Z 4Z 44 25 43 L/ 4 2/ 2/ fig: f1 9.' /Z 1 W-Zmt- V W j i l/7n 36-37 35 33 ffy' i /Cjgr /4 B G ntf G "700 fj, f 7m 66D/7ppanenA/aflbf/'b' 1600 5.6 Half, 1 lp /wgo nyu/0f "5" e/z waff; '500 lum ,40Z/zolgoarmA/r/Ms vlfw /2000 IODOO [0000 sooo ma 4D D /20 [60 290 240 260O M0 00 i200 /609 tm Z400 'fr ue Waffe /gaparen Neff: WITNESSES:INVENTOR Hen/y 1./ Puf/nan ATTORNEY Patented Nov. 3, 1942 MAGNETIC CORESTRUCTURE Henry V. Putman,

Sharon, Pas, assignorto Westinghouse Electric a Manufacturing Company,

a corporation of Pennsyl- East Pittsburgh, `Pa.,

vania Application January 29, 1941, Serial N0. 376,406

6Claims.

My invention relates to magnetic core structures for use in electricalinduction apparatus such as transformers.

In electrical induction apparatus, such as distribution or powertransformers, it has been the usual practice to employ a core structureof magnetic material formed of a stack of layers of thin sheets orlaminations of magnetic material such, for example, as hot-rolledsilicon steel. A portion of the magnetic structure is employed as awinding leg about which the copper circuit conducting windings of thetransformer are wound. In order to keep the cost of the transformers aslow as possible, it is desirable that the length of the mean turn of thecurrent carrying conductor about the winding leg -be as small aspracticably consistent with permissible losses and power performance ofthe transformer.

The smaller the cross-section of the magnetic material the greater willbe the flux density necessary to develop a given electromotive force andthe greater will be the exciting current flowing in the primary winding.The permissible exciting current for a given size of transformer hasbeen a factor in limiting the permissible reduction in the cross-sectionof the magnetic material.

The exciting current is made up of two components or parts, namely, themagnetizing current that is necessary to force the lines of magneticflux through the iron circuit of the core, and the iron loss currentnecessary to supply losses of energy in the iron core structure, such asthose due to hysteresis and eddy current losses. The component ofmagnetizing current is controlled very largely by the permeability ofthe material used in the core structure, and the watts loss component ofcurrent is controlled by the iron loss characteristic of the material.

The magnetic iron or steel usually employed in transformer corestructures is a high grade hotrolled silicon steel having a value ofpermeability and watts loss per cubic volume of substantially constantvalue, whether the lines of magnetizing flux pass through the steel inthe direction of rolling, or pass in other directions up to aconsiderable departure from the direction of rolling. More expensivesteels, such as Hipersil, have been developed having much higherpermeability and much lower losses in the direction of rolling. In suchmaterial the permeability varies considerably as the direction of thelines of magnetizing force vary from the direction of rolling of thematerial and is the highest in the direction of rolling. The watts lossper cubic volume or per cubic weight of the material likewise variesmarkedly as the direction of magnetizing flux varies from the directionof rolling of the material and is lowest in the direction of rolling.

The permeability of the new steel in the direction of rolling isconsiderably higher at operating densities (say 13 to 15 kilogausses persquare centimeter) than the permeability of commercial grades vofhot-rolled silicon steel at the same densities. Likewise the watts lossper unit volume or unit weight at operating densities when magnetized inthe direction of rolling is lower than for commercial grades ofhot-rolled silicon steel at the same density.

It will be seen, therefore, that if a transformer core is formed ofsheet steel having a preferred or greater permeability and a preferredor lesser watts loss per unit volume when the flow of flux is throughthe steel in the direction of rolling than when it is through thematerial in other directions and the sheets are so arranged that thelines of magnetic flux pass through the steel in the direction ofrolling, a higher permeability and lower watts loss will result,permitting the use of higher flux densities than are pennissible withthe usual grade of silicon steel, and a reduction of the cross-sectionof the magnetic material otherwise required.

Magnetic cores have been built with a high grade steel having apreferred permeability and low watts loss in the direction of rolling inwhich the core structure is formed by winding a ribbon of the steel toform loops in which the lines of magnetic force travel in the directionof the ribbon. There are, however, certain applications in which it isdesirable to form the core from stacks of sheets of laminated material,particularly in the larger sizes of core structures in which thecontinuous ribbon loops are not so readily adaptable.

It is an object of the invention to provide an electrical inductionapparatus having a core structure made up of stacked sheets of steelhaving a preferred permeability or easiest magnetization in thedirection of rolling so positioned that the lines of magnetizationcorrespond substantially to the direction of rolling of the material.

It is a further object of my invention to provide a core structure ofthe above indicated character employing combination butt and lap jointsadjacent the corners of a rectangular core structure in which themeeting edges are cut at an angle to the direction of the sheets toeffect a high permeability of the core structure as a whole. and as lowan iron loss as possible.

Other objects and advantages of the invention will be apparent from thefollowing description of certain embodiments thereof, reference beinghad to the accompanying drawings, in which:

Figures l and 2 illustrate the manner in which magnetic flux passesthrough core members of different designs;

Figs. 3 to 7, inclusive, illustrate parts of the core structures ofsingle phase transformer cores organized in accordance with theinvention;

Figs. 8 to 12, inclusive, illustrate parts of a three-phase corestructure; and

Figs. 13 and 14 show curves illustrating the comparative losses in twotypes of structures.

Fig. 1 illustrates a conventional arrangement of the laminatlons orsheet steel punchings commonly used in a core structure built from highgrade hot-rolled silicon steel comprising in each layer of the corestructure four punchings I, 2, I and l, arranged in the same plane, thepunchings I and 3 being alike and the punchings 2 and I being alike. Theseveral punchings are rectangular in shape, the punching I extendingfrom Ia to la', the punching 2 extending from 2a to 2a', the punching 3extending from 3a to la', and the punching 4 extending from 4a to la',so that one end of each of the four punchings is adjacent the side ofits adjoining punching. In this form of construction, position of thefour punchings will be shifted in the adjacent layer to provide anoverlap between the punchings in the adjacent layers so that thepunching I will extend from Ib to Ib', the punching 2 from 2b to 2b',the punching 3 from 3b to 3b', and the punching 4 from 4b to 4b', 4thepunchings in the second described layer forming the same general patternas those in the rst described layer but with the positions of theadjoining edges shifted from the full line locations Ia, 2a, 3a and lato the dotted line positions Ib', 2b', 3b and 4b.

In the structure shown in Fig. 1, the core legs 2 and l may be windinglegs of the core structure about which the primary and secondarywindings of the transformer are placed and the connecting portions I and3 form the yokes connecting the winding legs to complete the magneticcircuit of the core structure. In this structure the lines of magneticiiux pass lengthwise through the straight lportions of the legs andyoke, and flow about the corners of the structure something after thefashion indicated by the arrowed lines 8, so that near the ends of thepunchings the flux must pass sidewise of the members across the grain ofthe steel. This construction is entirely satisfactory for coresernploying conventional hot-rolled steel where the permeability is notmuch different at right-angles to the direction of rolling from thepermeability in the direction of rolling, but experience has shown'thatit is unsuited to the use of the new type steel having a much greaterpreferred permeability and lower watts loss when the lines of magneticflux pass through the material in the direction of rolling than whenthese lines of flux pass through the material in other directions.

'For such new type steel, in order to make efcient use of the propertiesof the material, it is necessary to employ a construction in which thelines of flux follow the direction of rolling, or easiest magnetization,at all points. In Fig. 2 of the drawings it will be noted that theadjoining edges of the several sheets 1. 8, 9 and III comprising a layerof the core material are cut on a diagonal or at a angle to thedirection of the sheets. In this core the flux will tend to follow thedirection of easiest magnetization, as shown by the arrow lines II,although this may not be the shortest path around the corner. Byfollowing this path of easiest magnetization or least resistance, thelines of flux avoid the necessity of passing crosswlse to the directionof rolling near the ends of the sheets of each lamination.

The merit of the construction in which the adjoining edges of thelaminations are cut at a diagonal to the direction of the sheet afterthe principle illustrated in Fig. 2, as compared to the conventionalbutt and lap joint illustrated in Fig. l, is readily shown by referenceto the curves in Figs. 13 and 14. In Fig. 13, the values for true wattsloss are plotted as abscissa for vary ing flux densities, plotted asordinates and indicated as kilogausses per square centimeter. Curve Ishows these values as obtained from tests on a core having anglar cutjoints corresponding to the principle illustrated in Fig. 2, and curveII shows the values obtained from tests on a core having the butt andlap joint de sign shown in Fig. 1, the two cores being identicalconstructions, except for the different manner of making the joints.These cores were of the same identical material, that is, the new steelabove referred to having the easiest magnetization in the direction ofrolling, which is the direction of the sheets or laminations as assembled in the core. It will be noted, for example, that, at a fluxdensity of 15 kilogausses per square centimeter, the core loss lfor theangular cut joint construction shown in Fig. 2 is 156 watts, and for theconstruction shown in Fig. l the core loss is 212 Watts, so that theFig. 2 construction has a watts loss that is TS1/2% of that for the Fig.1 construction.

Fig. 14 shows two curves, III and IV, similar to Fig. 13, except thatthe abscissa represents apparent watts, including both the power andreactive losses in the core, or the exciting voltampere characteristicof the steel. The apparent watts is considerably larger than the truewatts because the wattless component of exciting current is relativelylarge. The wattless component is also more sensitive to differences influx density. In Fig. 14 the curve III represents the voltamperecharacteristic of the core having the diagonal or angular cut joints asin Fig. 2, and the curve IV shows the volt-ampere characteristic of asimilar core except built with the lapped I-plate construction shown inFig. l, the two curves being for the same two cores as are representedby the curves in Fig. 13. It will be noted from Fig. 14 that at a fluxdensity of 15 kilogausses the apparent watts for the I-plate design ofFig. 1 is 2120 and the apparent watts for the Fig. 2 angular cut jointdesign is 660 or 31% oi' the apparent watts for the Fig. 1 construction.

This principle is made use of in the practical core structures shown inthe following figures and the punchings are so arranged that theadjoining edges in different layers are so shifted as to form acombination butt and lap joint having a lower joint reluctance than ifall the layers were built as in Fig. 2 in which the diagonal linesforming the junctions between the layers would be superimposed upon eachother.

Referring to the core structure shown in Fig. 3 comprising fourpunchings I4, I5, I6 and Il in each layer of the core, the punchings I 4and Il are alike and are shown in detail in Fig. 5, and

the punchings Il 1nd I1 are alike and are shown in detail in Fig. 4.Thearrangement oi' the four members oi' the core structure, that is, thetwo winding leg members made up from stacks of laminations I and I1, andthe yoke members made up from stacks of laminations I4 and Il, form awindow having a width W and a height L. The width oi' the laminations I4and I9 is slightly greater than that of the laminations I5 and I1, thelatter being indicated by the dimension N and the former by thedimension N-I-n. The inner edge of punchings I4 and I6, that isindicated in Fig. 5 as W-m, is likewise less than thewidth o! thewindow, the dimension m corresponding to the amount that the inner endof the diagonal edge of the punching shown in Fig. 5 is oil'set from theadjacent window corner as indicated in Fig. 3. It will also be notedthat the inner edge oi' the punchings I5 and I1 is somewhat longer thanthe length of the window as indicated in Fig. 4 by the dimension L-I-n.Referring to the layer represented by full line positions of thepunchings I4, Il, I6 and I1 in Fig. 3, it will be noted that theadjoining edges between laminations I8 and I1 extend i'rom a point I 9that is offset from the outer corner of the core member by the dimensionn to the point I9 corresponding to the corner of the window, and that atthe opposite end of the punching I8 the adjoining edges betweenlaminations I 5 and I6 extend from the point 2| at the outer corner ofthe core structure to a point 22 that is offset from the inner corner ofthe window by the dimension m. It will also be noted that the diagonallyopposite corners are alike, that is to say, that in the upper right andlower left corner of the structure as shown in Fig. 3, the junctionbetween the adjacent edges of the laminations shown in full linesextends outwardly from the corner of the window to a point oil'set fromthe outer comer of the structure, while in the lower right and upperlei't corners of the structure the adjoining edges of the laminationsextend from the outer corner of the structure to a point oil'set fromthe inner corner of the window. It will also be noted that in theadjacent layer in which the adjoining edges between the laminations inthe same layer are indicated in dotted lines, the locations of theadjoining edges are reversed so that at each corner where the junctionof one layer starts outwardly from the corner of the window, thejunction of the next layer starts inwardly from the outer corner of thestructure. In effecting this reversal in the location of the adjoiningedges between the laminations of a layer, the punchings I4, I6, as shownin Fig. 5, will be turned end for end so that the offset portion shownby the dimension n at the lower lei't of Fig. 5 will, in an adjacentlayer of laminations, be at the lower right in Fig. 3 and will likewisealternate from the upper right to upper left position for the laminationI4. The punchings I 5 and I1 are alike at both ends and are shiftedlengthwise in adjacent layers an amount indicated by the dimension n, sothat for the punching I5, for example, in the first described layerrepresented in full lines in Fig. 3, this member will extend from pointsI9 and I9 at the upper end to the points 2| and 23 at the lower end,while in the second described layer of laminations indicated in dottedlines it will extend from the points 2I and 23 at the upper end topoints I8 and I9 at the lower end. The successive layers of thelamination I1 vary in the same manner as for IB, the lamination I1 beingin its lower position in the layer 75 3 in which the lamination I5 is inits higher position and vice versa.

Fig. 6 shows an arrangement of laminations 24, 2B, 29 and 21 which is,in general, similar to that shown in Fig. 3 and in which punchings 2Band 21 correspond to that outlined in Fig. 4. and the punchings 24 and29 correspond to that outlined in Fig. '1. The punching shown in Fig. 7diilers from that shown in Fig. 5 in that the inner edges of thepunchings 24 and 29 are of a length W corresponding to the full width ofthe window and extend at right angles for a dimension n so that thesloping portion p is the same at both ends of the punching correspondingto the dimension p of the diagonal edge of the punching shown in Fig. 4.With this arrangement of the upper and lower punchings which are used tobuild up the yoke portions of the core the small voids represented bythe triangles I9, 22, 23 in Fig. 3 are done away with and the corestructure presents a smooth inner surface about the comers of thewindow.

It will be noted that the constructions shown in Figs. 3 and 6 are suchthat although the acijacent edges of the sheets in the several punchingsof adjacent layers or laminations of the structure are staggered so asto provide the butt and lap joints at the corners of the structure, theinner and outer edges of the layers oi punchings are in alignment, thatis, the inner edges of the punchings forming the window of the structureline up with the corresponding edges of the adjacent layers of themagnetic material,` and the outer edges of the several laminationscorrespondingly align so as to make a compact structure having theminimum dimensions for the given amount of material used. Thisconstruction makes it possible to tap or block the core, that is, toapply pressure to the edges of the sheets in the stack in order to getthe edges of all of the sheets lined up.

It will also be noted that the yoke members y above and below thewindows as shown in Fig. 8 have a width of N-I-n, whereas the verticalor winding leg members of the core have a width N so that the yokemembers are widened with respect to the winding leg members, that is,have a' greater cross-section than the winding leg members so that theflux density therein is less. This widening can be increased as desiredby changing the angle of cut between the adjacent members of the sheetsfrom that of as shown, without introducing any cross flux. The` fluxwill therefore follow the lines of easiest magnetization as shown by thearrowed lines II in Fig. 2.

It will also be noted that by cutting the adjoining edges of the sheetsof magnetic material to provide the angular cut joints shown in Figs. 2,3 and 6, the reluctance of the joint between adjacent sheets will belowered because the length of the adjoining edges is increased by theratio of the \/2 to 1 with respect to the right-angle cuts shown inFig. 1. This increase in the length of the adjoining edges betweenadjacent members causes the flux density at the joint to becorrespondingly reduced. This decrease in the reluctance oi the air gapin the diagonal butt and lapped joint shown in Figs. 3 and 6 greatlyincreases the efficiency of the core at the gap over that of a corehaving the conventional butt and lapped joints shown in Fig. i.

In Fig. 8 an arrangement of punchings is illustrated for building up acore having three legs such as might be used in a three-phasetransformer. The central leg 32 is built up from punchings having theoutline shown in Fig. 9 and the outer legs 33 and 3l from punchingshaving the outline shown in Fig. 10, while the yoke members 35, 33, 3land 33 are built up or punchings shaped as shown in Figs. il and 12. Theouter leg members 33 and 34 alternately change their position indifferent layers of the core structure in the same manner as the twoouter leg punchings I6 and I1 in Fig. 3 extend- Ing from points 2l and23 at the top to points Il and I3 at the bottom in one layer, as shownin full lines for the member 3|, and from the points I3 and I9 at thetop to points 2I and 23 at the bottom as shown in dotted lines for themember 3l in Fig. 8. 'I'he outer edges of the member` 32, like the inneredges of the members 33 and 3l are longer than the windows in the corestructure by the dimension n, the corner 42 on one edge being advancedby the dimension n lengthwise beyond the corner II on the opposite side.The slopes of the edges from the corners 3| and I2 to the points I3 atthe ends of the members 32 are at an angle of 45 degrees to thedirection o! the member 32. By placing the members 32 in the core so asto alternately vary the side that is uppermost, the end point 43 variesits position from point 43 to point M as shown in Fig. 8 and the pointsII and 42 shift sides. The point Il lcoincides with the corner of thewindow and the point I2 is a distance n vertically distant therefrom.Two diil'erent shaped punchings are required for the yoke pieces asshown in Fig. 11 and 12, the two members alternating from side to side..For example, the member in Fig. 11 is shown in full lines at 31, and 36in Fig. 8, and the member shown in Fig. 12 is shown inv iull lines at 35and 38 in Fig. 8. In the adjacent layer positions, the relativepositions of the members are reversed so that the member in Fig. 11 isat 35 and 3B as shown in dotted lines and the member in Fig. 12 is at 31and 36 as shown in dotted lines. It will be noted that In Fig. 12 thepunching on its inner side is the same length W as the width of thewindows, and the edges adjoining adjacent punchings in the same layerextend outwardly from the inner corner oi the window to a point onsetfrom the corner of the punching or core structure by the dimension n.I'he punching shown in Fig. 11 has a dimension on its inner edge ofW-2m, so that when placed in position as shown in the full lines in Fig.8 as yoke members 31 and 36, both inner corners are oIIset irom thecorners ot the window by the dimension 1n, and the edge adjoining theadjacent outer leg punching in the same layer extends from its offsetpoint to the outer corner of the core structure.

In the full outline of the sheets shown in Fig. 8, the outer leg member33 is shown in its lower position extending from the points I8 and I9 atthe top to points 2I and 23 at the bottom, while the leg 3l is in itsupper position extending from points 2i and 23 at the top to points I9and I8 at the bottom, and the center leg member 32 is in a position inwhich the corners 42 are at the upper right and lower left of themember, as viewed in Figs. 8 and 9, and the corners 4I are at the upperleft and lower right as viewed in these figures. Also the punching shownin Fig. l1 is shown in the full line position in Fig. 8 as at the bottomof the left window and at the top of the right window, while thepunching shown in Fig. 12 is shown at the top of the left window and atthe bottom of the right window. In the layer of laminations having theiradjoining edges shown in dotted lines, the positions ot the severalpunchings are shifted from the full line positions, the punching in theieg 33 being shown in the upper position and the punching in the leg 34being shown in its lower position, while in the middle leg 32 theposition of the punching is reversed so that the end points 43, as shownin Fig. 9, occupy the positions Il as shown in Fig. 8, and the cornersI2 are in the upper left and lower right corners of the punching asviewed in Fig. 8. Likewise the positions of the punchings shown in Figs.11 and 12 are reversed, the punching of Fig. 11 being shown in dottedlines below the right-hand window and above the left-hand window, andthe punching of Fig. 12 being shown in dotted lines below the left-handwindow and above the right-hand window. This arrangement of thealternate layers oi' punchings used to build up the core structureprovides an arrangement in which the several sheets or laminationsemployed are so positioned in the structure that the lines of magneticilux extend lengthwise of the members up to the adjoining edge ci' thenext adjacent member in the same layer of punchings comprising the core,and the adjoining edges of the punchings in adjacent layers are ofl'setfromeach other to provide a diagonal overlap adjacent the corners oi'the windows in a manner similar to the arrangements shown in Figs. 3 and4.

It will be apparent to those skilled in the art that many modiilcationsin the detailed arrangements oi the parts of the structure may be madewithin the spirit of my invention and I do not wish to be limitedotherwise than by the scope of the appended claims.

I claim as my invention:

l. In an electrical apparatus, a substantially rectangular magnetic corestructure built up from layers of sheets of magnetic material formed ofsteel having preferred greater permeability and lower watts loss in thedirection of rolling than in other directions and so positioned in thestructure that the direction of magnetization is substantiallycoincident with the direction of rolling.I each layer comprising aplurality of sheets having adjoining edges adjacent the corners of thecore structure and arranged about a substantially rectangular window,the junctions of all adjoining edges at the outer corners of themagnetic core extending along continuous straight lines at anglesoblique to the direction oi' rolling so that substantially all iiux ineach sheet passes between junctions at opposite ends of each sheet in acontinuous straight path along the direction of rolling, the line ofjunction be tween said adjoining edges of the sheets of magneticmaterial of one layer at any corner of the window extending outwardlyfrom the inner angle of the window. and the line of junction between theadjoining edges of any adjacent layer at the same corner extendingoutwardly :from a point offset from the inner angle of the window at anangle to provide an overlap of substantially constant width between theadjacent junction lines of said adjacent layers.

2. In an electrical apparatus, a substantially rectangular magnetic corestructure built up from layers of sheets of magnetic material formed ofsteel having preferred magnetic properties in the direction of rollingand so positioned in the structure that the direction of magnetizationis substantially coincident with the direction of rolling. each layercomprising a plurality of sheets having adjoining edges adjacent thecorners of the core structure and arranged about a substantiallyrectangular window, the junctions of all adjoining edges at the outercorners of the magnetic core extending along continuous straight linesat angles oblique tothe direction of rolling so that substantially allflux in each sheet passes between junctions at opposite ends of eachsheet in a continuous straight path along the direction of rolling, theline of junction between said adjoining edges of the sheets of one layerat a pair of alternate corners of the core extending from the innerangle of the window,

the line of junction between the adjoining edges of said layer at theother pair of alternate corners of the core extending from a pointoffset from the inner angle of the window, the sheets in any adjacentlayer being so arranged that the points vfrom which the lines ofjunction extend are the reverse of those of the first-named layer, thejunction lines inthe several adjacent layers at any one corner of thecore extending in parallel directions to provide an overlap ofsubstantially constant width between the junction lines of the adjacentlayers.

3. In an' electrical apparatus, a magnetic core structure built up fromlayers of sheets of magnetic material formed of steel having preferredmagnetic properties in the direction of rolling and so positioned in thestructure that the direction of magnetization is substantiallycoincident with the direction of rolling, each layer comprising aplurality of sheets arranged to'form a substantially rectangular window,the junctions of all adjoining edges of any of the sheets in any layerextending along continuous straight lines at angles oblique to thedirection of rolling so that substantially all flux in each sheet passesbetween junctions at opposite ends of each sheet in a continuousstraight path along the direction of rolling, the sheets on two oppositesides of said window being of a diierent width than the sheets on theother two opposite sides ofthe window, the lines of junction between theadjoining edges of the sheets comprising one layer being arranged at twodiagonally opposite corners of the window to extend from points olsetfrom the outer corners of the structure along continuous straight linesto the inner corners of the windows, and the lines of junction betweenthe adjoining edges of the sheets at the two remaining diagonallyopposite corners in the same layer extending from the outer corners ofthe structure along continuous straight lines to points offset from thecorners of the Window, the location of the lines of junction between alladjoining edges being reversed in adjacent layers between the two setsof diagonally opposite corners to provide an overlap of substantiallyconstant width between the adjacent lines of junction of the adjacentlayers.

4. In an electrical apparatus, a magnetic core structure comprising legmembers and yoke members at angles of substantially 90 to each other toform a closed magnetic circuit about a rectangular window, the leg andyoke members being built up of layers of sheet members formed of steelhaving preferred magnetic properties in the direction of rolling and sopositioned in the structure that the direction of magnetization issubstantially coincident with the direction of rolling, each layercomprising a plurality of sheets having adjoining edges between leg andyoke members adjacent the corners of the corel the sheets in the severallayers having their inner and outer lineal edges in alignment and thejunctions of all adjoining edges between the sheets at substantially 45to the lineal edges of the members so that substantially all flux ineach sheet passes between junctions vat opposite ends of each sheetalong a continuous straight path in the direction of rolling, thejunctions of all the adjoining edges of said sheet members in a givenlayer being oiset from, and parallel to, the junctions of the adjoiningedges of the sheet members in an adjacent layer at the correspondingcorners of the core to provide an overlap of substantially constantwidth between the adjacent junction lines of the adjacent layers, saidyoke members being of a dilerent width than the leg members, the membersof the greater width having the ends adjoining adjacent members in thesame layer offset dierently at the opposite ends thereof with respect tothe corners of the structure.

5. In an electrical apparatus, a magnetic core structure built up from aplurality of layers of sheets of magnetic material formed of steelhaving preferredrmagnetic properties in the` direction of rolling and sopositioned in the structure that the direction of magnetization issubstantially coincident with the direction of rolling, said corestructure comprising three winding leg members and yoke membersconnecting the ends of the leg members to form a substantiallyrectangular core having two substantially rectangular windows, thejunctions of all adjoining edges between the sheets of the leg and yokemembers in any layer at the outer corners of the core extending alongcontinuous straight lines at oblique angles to the direction of thesheets so that substantially all iux in each sheet passes betweenjunctions at opposite ends of each sheet in a continuous straight pathalong the direction of rolling, the line of junction between theadjoining edges of the sheets of one layer at a cor'' responding end ofboth windows extending outwardly from the inner angles of the windowsand the line of junction between the adjoining edges of the sheets ofthe same layer at the other ends of the windows extending outwardly frompoints olset along the yoke from the inner angles of the windows, thelines vof junction between the adjoining edges of the sheets indifferent layers at any corner extending in parallel lines outwardlyfrom the corner, the positions of the parallel lines being reversed inadjacent layers to provide an overlap of substantially constant widthbetween the adjacent junction lilies of the adjacent layers.

6. In an electrical apparatus, a magnetic core structure built up from aplurality of layers of sheets of magnetic material formed of steelhaving preferred magnetic properties in the direction of rolling and sopositioned in the structure that the direction of magnetization issubstantially coincident with the direction of rolling, said corestructure comprising three leg members and yoke members connecting theends of the leg members to iorm a substantially rectangular core havingtwo substantially rectangular windows, the junctions oi' all adjoiningedges between the sheets forming the leg and yoke members in any layerat the outer corners of the core extending along continuous straightlines at oblique angles to the direction of the sheets so thatsubstantially all iiux in each sheet passes between junctions atopposite ends of each sheet in a continuous straight path along thedirection o1' rolling, the lines of junction between all the adtion o!the core having their inner edges o! a length less than the width of thewindow. the sheets in adjacent layers of the yoke portion havingdiierent lengths of the inner edges so that the junction lines in theseveral adjacent layers at any one corner of the core extend in paralleldirections to provide an overlap of substantially constant width betweenthe Junction lines oi the adjacent layers.

HENRY V. PU'IMAN.

